1. Technical Field
The present invention relates to a photoelectric conversion element formed with a thin film of a photoelectric absorption material such as polysilicon film or amorphous silicon film, a solid-state imaging element or detector such as image sensor, solar cell and X-ray flat panel sensor, and a system such as liquid crystal display, biometric personal authentication system, photovoltaic power generation system, X-ray imaging system and digital camera using the same.
2. Background Art
In photo-detectors for visible light, crystalline silicon has mainly been used so far to date. However, use of thin-film sensors using amorphous silicon or polysilicon is spreading in recent years.
Amorphous silicon films are also used in, besides photo-detectors, solar cells, X-ray flat panels and so forth. They can be formed as a semiconductor layer only for a necessary part on a substrate, which is formed with a relatively inexpensive material such as glass and plastics. It is easy to obtain such films of larger area, and the manufacturing process thereof is also relatively easy. Therefore, they have an advantage that they can be made more inexpensive compared with those consisting of single crystal or polycrystal silicon. However, they have a problem that the detection efficiency and efficiency for conversion into electric signals are lower than those of crystalline silicon. Moreover, crystalline silicon shows the peak sensitivity at a wavelength of around 800 nm in the near-infrared region and shows sensitivity up to around 1100 nm, whereas amorphous silicon shows the peak sensitivity at around 600 nm. Therefore, amorphous silicon shows low sensitivity especially for light of the near-infrared region. Accordingly, such a characteristic poses a problem when amorphous silicon is used for a photo-detector for that wavelength.
If it is attempted to solve this problem by using a thicker amorphous silicon, a high technical level is required to uniformly form a thick film, and in addition, there arise problems such as degradation of the production yield, longer lamination time and more serious difficulty in forming finer patterns, which may result in factors of higher cost.
On the other hand, polysilicon is widely used in liquid crystal displays using pixel thin film transistors. Furthermore, photo-detectors incorporated into liquid crystal displays and so forth have been developed in recent years. Such use of polysilicon as described above have an advantage that a photo-detector can be loaded together on other systems such as liquid crystal displays to make it possible to use common peripheral circuits for them, and a circuit such as amplifier can be incorporated into a sensor. However, polysilicon films have a problem that they show the peak sensitivity around the blue light region of a wavelength of 450 to 500 nm, and scarcely show sensitivity for light of from the red region to the near-infrared region. This is especially because of the thin film thickness. Polysilicon films are usually obtained by melting and solidifying amorphous films with irradiation of excimer laser light, and thickness of amorphous silicon film is limited to the thickness through which excimer laser light can penetrate. The upper limit is about 50 nm as described in, for example, Patent document 3.
As an effective method for improving detection sensitivity of these materials suffering from a limitation on use of thicker films, there is a method of returning photons not absorbed by a semiconductor layer to the semiconductor layer again by reflection with a reflecting layer provided on the side opposite to the photon incidence side to make them to be absorbed by the semiconductor layer, and a method for manufacturing an efficient reflecting layer is proposed in Patent document 1. Moreover, in order to realize unevenness of 100 nm or more effective for reflecting photons for a reflecting layer, Patent document 2 proposes a structure in which a transparent substrate is provided between a photoelectric conversion film and a reflecting layer.
Furthermore, Patent document 3 proposes a method of forming a photoelectric conversion film made of polysilicon in a concavo-convex shape so that photons should enter obliquely to cause multiple scattering in the photoelectric conversion film and thereby improve the sensitivity for visible light.
[Patent document 1] Japanese Patent Unexamined Publication (KOKAI) No. 2001-100389
[Patent document 2] Japanese Patent Unexamined Publication (KOKAI) No. 5-218469
[Patent document 3] Japanese Patent Unexamined Publication (KOKAI) No. 2004-247686